1. Field of the Invention
The invention comprises a control for an internal combustion engine in a vehicle powertrain wherein the response time for a driver demand for driving torque is reduced and engine start smoothness is improved.
2. Background Art
Hybrid electric vehicle powertrains of known design can be classified generally into three main categories commonly referred to as series hybrid powertrains, parallel hybrid powertrains and series-parallel hybrid powertrains. In each case, two power sources are available for powering a driven element connected driveably to vehicle traction wheels.
A series-hybrid powertrain comprises a fueled engine prime mover, which powers an electric or a hydraulic power transmission connected to a drive motor. The motor can be driven by a battery or by an engine driven generator. A parallel hybrid electric vehicle powertrain establishes parallel power flow paths from the engine through power transmission gearing as stored electrical energy drives the driven member through power transmission gearing. A so-called parallel-series hybrid electric vehicle powertrain combines a series-hybrid function and a parallel hybrid function. A parallel-series powertrain is disclosed in U.S. patent application Ser. No. 10/709,537, filed May 12, 2004, now U.S. Pat No. 7,013,213,and in U.S. patent application Ser. No. 10/905,324, filed Dec. 28, 2004. This patent and this patent application are assigned to the assignee of the present invention.
Parallel-series hybrid electric vehicle powertrains provide power flow paths to vehicle traction wheels through gearing. In one operating mode, a combination of an internal combustion engine and an electric motor-generator subsystem define in part separate torque delivery paths. The motor-generator subsystem includes a battery, which acts as an energy storing medium. In a first forward driving mode, the engine propels the vehicle using reaction torque of a generator, which is a part of the motor-generator subsystem. Planetary gearing makes it possible for the engine speed to be controlled independently of vehicle speed using generator speed control. In this configuration, engine power is divided between a mechanical power flow path and an electrical power flow path. The generator is electrically coupled to an electric motor of the motor-generator subsystem, which in turn drives the vehicle traction wheels. Because the engine speed is decoupled from the vehicle speed, the powertrain emulates the characteristics of a continuously variable transmission during a driving mode in which the engine is active.
The electric motor provides a braking torque to capture vehicle kinetic energy during braking, thus charging the battery as the motor acts as a generator. Further, the generator, using battery power, can drive against a one-way clutch on the engine power output shaft to propel the vehicle in a forward drive mode as the generator acts as a motor.
As in the case of conventional continuously variable transmissions in vehicle powertrains, it is possible to achieve better fuel economy and exhaust gas emission quality by operating the engine at or near the most efficient operating region of its engine speed and torque relationship. The engine can be stopped if the engine operating conditions are not favorable for high fuel efficiency operation or if the engine is not in a high emission quality operating region. In this way, the two power sources (i.e., the engine and the motor-generator subsystem) can be integrated and coordinated to work together seamlessly to achieve better fuel economy and emissions control.
A vehicle system controller performs the coordination of the control of the two power sources. Under normal powertrain conditions, the vehicle system controller interprets a driver demand for acceleration or deceleration torque and then determines when and how much torque each power source needs to provide in order to meet the driver's demand and achieve specified vehicle performance. Specifically, the vehicle system controller determines the speed and torque operating point for the engine.
The internal combustion engine, during an engine cranking mode during engine start ups, has an engine throttle position that is set to a fixed crank position. This position typically is very small (e.g., 1–2°) while the engine speed is increased up to the desired cranking speed and initial fuel injection takes place. Typically, the engine would include an electronic throttle with a controller that establishes an optimum fixed throttle angle during engine cranking, followed by an initial engine torque command position at the instant the engine running mode is initiated. At that instant, the control of the electronic throttle switches from a cranking software logic to a torque-based software logic for engine torque control. The throttle position effectively is fixed at a constant angle by the cranking logic, which ensures sufficient air flow through the engine throttle body to overcome engine frictional losses during initial engine combustion. Engine fuel injectors initiate fuel supply as combustion is started. Once combustion is established, control of the electronic throttle switches, from the cranking software logic to the engine torque control software logic. The cranking angle is independent of the target torque after the engine starts. Further, the initial engine torque command at the initiation of engine fueling is also independent of the target torque after the engine is running.
To achieve smooth engine starts at low power demand, the cranking throttle position should be relatively small, which results in a manifold pressure that is reduced to a low level during an engine start mode. If a high engine power is desired after the engine starts, the manifold pressure must be re-established and increased from the low engine cranking pressure value to a value consistent with the higher engine power that is desired. The re-establishment of manifold pressure delays the response time of the engine.
Merely adjusting the crank throttle position based on power demand or commanded torque would not be sufficient to improve response time since the commanded torque must be changed smoothly from an initial engine torque command to the desired, or targeted, engine torque command. If the initial torque command is too low (i.e., lower than the torque produced at the crank throttle angle), then the throttle position may initially close after the engine start. It then would be re-opened as the engine torque command increases to the target engine torque. Further, the smooth increase of the commanded engine torque from the initial engine torque to the desired engine torque may be too slow.